Many test systems need to add inter symbol interference (ISI) as a stress source when testing serial digital receivers. This is done to emulate real-life situations where long cables and/or printed circuit board (PCB) traces add ISI into a communications link. Once added, the receiver is challenged to operate in error-free conditions despite the added impairing stress. Grading mechanisms for receivers amount to seeing how much ISI a given receiver can operate error-free with. For this reason, creating a continuously variable amount of ISI in a test system is desirable.
Continuously variable ISI has been elusive as the current state of the art employs RF switches to switch-in different lengths of PCB traces which causes a practical limitation to the number of trace-length paths that can be inserted. This decreases the resolution of the varying increments of ISI. Alternatively, a large collection of fixed PCB traces can be used and manually cabled; however, this presents a problem for automatically changing between one amount of ISI stress and another.
What is needed is a variable ISI generator.